A giant 2-dimensional dielectric response in a compressed hydrogen-bonded hybrid organic–inorganic salt

Abstract

A strongly anisotropic 2-dimensional relaxor-like dielectric response has been obtained in the hybrid organic–inorganic crystal of pyrazinium tetrachloroaurate [C₄H₅N₂]⁺AuCl₄⁻, composed of pyrazinium aggregates separated by AuCl₄⁻ polyanionic sheets. This crystal is stoichiometric and it is the first known structure to exhibit 2D relaxor-like properties. This anisotropic response stems from the highly polarizable hydrogen bonds NH⁺⋯N linking the pyrazinium cations into chains extending in diagonal directions perpendicular to the tetragonal axis. Under normal conditions, the NH⁺⋯N bond is long, but high pressure reduces the N⋯N distance from 2.867 to 2.41 Å at 12.67 GPa. A pressure of only 1 GPa results in a huge increase of the electric permittivity by two orders of magnitude within the plane parallel to the NH⁺⋯N bonds. This is an emerging new class of layered stoichiometric materials with a large anisotropic dielectric constant highly sensitive to external stimuli.